Superlative form meaning the most surprising, peculiar, or frustrating; used informally to describe something extremely odd or annoying.
From durn with the superlative suffix -est, following standard English pattern. The formation parallels 'darnedest,' creating a mild, family-friendly superlative.
'The durndest thing' is a wonderfully folksy expression—you might hear 'that's the durndest thing I ever saw' from a rural American grandmother describing something genuinely bizarre, and it conveys judgment wrapped in affection.
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